1. Field of the Invention
The invention relates to an actuator for an automatic transmission provided with electrical actuation, preferably for automobiles.
The solution according to the invention is suitable for automatic transmissions, especially for automobiles which are often parked on inclined surfaces in the driving direction.
2. Discussion of the Prior Art
The object of the selection actuation in an automobile is to convey the gear setting adjusted by the driver to the transmission. It is possible to do this mechanically or electrically. With electrical selection actuation in the driver's compartment of the vehicle, the selection actuation is only an electric switch or sensor which conveys the gear selected by the driver to the transmission controller. The selection actuation can be constructed as a conventional lever. However, it can also be realized as a rotary switch or by means of push buttons.
The transmission controller contains a microprocessor whose function it is to compare the gear position of the transmission with that of the selection actuation and, in the event of a difference, to control the actuator at the transmission so that the transmission and selection actuation have the same gear speed again.
In addition, the system has a display in the driver's compartment of the vehicle which displays the selected gear speed to the driver.
An automatic transmission has at least gears “P-R-N-D”, and can also provide gears “L” or “3, 2, 1”.
The use of electrically actuated automatic transmissions is less common, but has been known since the 1970s.
When a vehicle with automatic transmission is parked on a hill, the mechanical locking of the transmission in the “P” gear position is tensioned by the descending force of the inclination. Therefore, a very high expenditure of energy is needed to remove the transmission from the “P” gear position. For vehicles with an electrically actuated automatic transmission, this means that the electric drive in question must be designed with a distinctly high output; however, as a result of this, the drive responds very sluggishly and is very heavy. Moreover, drives of this type occupy a large amount of space and are expensive. When shifting through from one end of the shift gate to the other, the rest of the shift positions of the transmission require only a very weak drive which responds very quickly, since shifting through should take no longer than a few seconds. Further, the positioning of the hydraulic piston of the automatic transmission must be carried out very precisely. A powerful and sluggish drive such as that in large electric motors is unsuitable for these tasks. This statement applies in principle to all drives or actuators.
A catch mechanism arranged in the transmission is used for precise positioning off the hydraulic cylinder, also known as the selector slide. Since the tolerance chain between selection actuation and transmission is too large for precise positioning with mechanical actuation, play is required between the drive and transmission. Otherwise, the exact positioning relative to the catch mechanism would have to be carried out by the actuator.
“PKW-Automatengetriebe”, Verlag Moderne Industrie AG, Landsberg/Lech 1994, pp. 24, 25, discloses a parking lock for automatic transmissions in which a detent catches in the parking ratchet wheel in the “P” position of the selector lever. One of the requirements made on this mechanism respecting operation and reliability is that the vehicle is held in forward and reverse also on steep grades and the lock can be disengaged again under these conditions.
This is problematic precisely with steep grades, e.g., garage entrances, with grades of up to 30 percent.
In order to meet these demands, U.S. Pat. No. 5,827,149 A describes an electrically controlled actuating device having two electric motors, wherein one electric motor adjusts the actuating shaft of the transmission and is provided with a rod linkage and the other motor serves to disengage the detent. The disadvantage of this arrangement is that no higher forces can be transmitted via the electric motor, which is a stepper motor. Further, two electric motors are required to disengage the detent and the parking ratchet wheel. This reference does not disclose a gear unit with variable transmission ratio between the drive and the automatic transmission.